The principle of optical heart rate measurement is based on the use of a light source that emits light into the skin of a user. Such a measurement is called photoplethysmography (PPG). The emitted light is scattered within the skin, where it is absorbed partially by blood. Reflected light exits the skin and is captured by a sensor. The amount of the signal on the sensor is an indication of the blood volume. The blood volume in the skin changes when the blood stream pulsates and, as a consequence, the signal on the sensor changes directly in response to the pulsation. Hence, the sensor measures directly a pulse of the user in the skin and can thus determine the actual heart rate of the user at a given moment.
The measurement of the heart rate becomes more complex when the user is in motion. Such a motion, even slight, generates motion artefact, which can be defined as a noise signal on top of the pulse signal. Therefore, in order to obtain a highly reliable heartbeat measurement, the motion artefact should be eliminated from the measured pulse signal.
European patent application EP1354553 discloses an apparatus for detecting a heartbeat by using photoplethysmography, and which is capable of detecting the heartbeat even under motion of a user. Such an apparatus comprises a filtering unit for selectively passing only a signal within a specific frequency band among the PPG signal detected by the PPG sensor unit. Such a solution is quite complex and might not always be reliable.